Yes, the pectic enzyme is a good idea - plums have a fair amount of pectin and the prunerisation(?) process seems to make that worse.

I think you may find that the flavour will turn out to be way over the top. The standard recipe for prune wine uses 4 lbs of fruit and I don't think you'd end up with 4 litres of juice from that. However, I don't know this, so maybe it'll turn out as one of the seven modern wonders of the world. I'd have been tempted to make two gallons from it, though, but don't take my word as gospel.